In recent years, an optical information recording system has widely been studied and developed in the field of information recording.
The optical information recording system has a number of advantages such as can deal with various memory types of non-contact type recording and reproduction, reproduction-only type, write-once type, and rewritable type; and therefore, from industrial use to consumer use, it is contemplated as a system capable of obtaining an inexpensive large-capacity file.
A large capacity of these various optical recording media for the optical information recording system, for example, an optical disc, has been accomplished mainly by making a wavelength of a laser beam, which is a light source used for the optical information recording system, short, as well as by using an objective lens having a high numerical aperture (N.A.) to make a spot size on a focal plane small.
For example, CD (Compact Disc) has a capacity of 650 MB when a wavelength of a laser beam is 780 nm and a numerical aperture (N.A.) of an objective lens is 0.45, whereas DVD-ROM (Digital Versatile Disc—Read Only Memory) has a capacity of 4.7 GB when a wavelength of a laser beam is 650 nm and a N.A. of an objective lens is 0.6.
Further, in an optical disc system of the next generation, by using an optical disc having a thin light-transmittable layer with a film thickness of, for example, approximately 0.1 mm formed on the optical recording layer, applying a laser beam from the side of the light-transmittable layer, and making a wavelength of a laser beam 450 nm or less and making N.A. 0.78 or more, a large capacity of 22 GB or more is made to be possible.
FIG. 4A is a schematic perspective view showing a state of optical recording or optical reproduction with respect to the optical disc.
An optical disc DC is a disc having a center hole CH bored in the center and is driven to rotate in the direction, for example, shown by an arrow mark DR in FIG. 4A.
FIG. 4B is a schematic section of the optical disc DC. FIG. 4C is an enlarged section of a relevant part of the optical disc DC.
This optical disc has the structure in which a concave portion 101r is formed on the main surface of a disc substrate 101 made of, for example, polycarbonate resin having a thickness of about 1.1 mm, and an optical recording layer 102 is formed along a concave and convex surface including the concave portion 101r. 
For example, in an optical disc DC of phase-change type, the optical recording layer 102 is made of a laminate of, for example, a dielectric film, a phase-change film, a dielectric film, a reflective film, and so on. Further, on the optical recording layer 102 is formed a light-transmittable layer 103 having a film thickness of, for example, 0.1 mm.
When information is recorded in or reproduced from the optical disc DC, light LT by a laser beam having a wavelength of 450 nm or less, for example, 380 nm to 420 nm is focused and applied to the optical recording layer 102 from the side of the light-transmittable layer 103 of the optical disc DC through an objective lens OL having a numerical aperture of 0.78 or more, for example, 0.85.
When the recorded information is reproduced, returned light reflected at the optical recording layer 102 is received by a light-receiving element and a signal-processing circuit generates a predetermined signal to provide a reproduced signal.
The optical recording layer 102 of this optical disc has a concave and convex shape resulted from the above-described concave portion 101r formed on the surface of the disc substrate 101.
The concave portion 101r forms, for example, a spiral continuous groove or grooves of concentric circles with a predetermined pitch, and with this concave and convex shape, a track area is divided.
The concave portion and convex portion of the concave and convex shape dividing the track area are called groove and land, respectively. When a land-groove recording method for recording information in both of the land and groove is employed, the large capacity can be obtained. However, only one of the land and groove can be made into a recording area.
In addition, for example, by making the concave and convex shape caused by the concave portion 101r formed on the disc substrate 101 into pits each having a length corresponding to recording data, a read-only-memory (ROM) type optical disc can be obtained.
It has been verified that tin oxide (SnOz, z<2) which is a metal oxide with non-stoichiometric composition can be employed as a recording material forming the optical recording layer (See Journal of Materials Science Letters 19, 2000. 1833-1835).
However, in this case, when information is recorded using an objective lens having a numerical aperture of about 0.8 and a laser beam having a short wavelength of about 380 nm-420 nm, such a problem is posed that no recording mark in a satisfactory shape is formed and the jitter increases.